Four layers are generally present in the construction of a conventional, prerecorded, optical disc. A first layer is usually made from optical grade, polycarbonate resin. This layer is manufactured by well-known techniques that usually begin by injection or compression molding the resin into a disc. The surface of the disc is molded or stamped with extremely small and precisely located pits and lands. These pits and lands have a predetermined size and, as explained below, are ultimately the vehicles for storing information on the disc.
After stamping, an optically reflective layer is placed over the information pits and lands. The reflective layer is usually made of aluminum or an aluminum alloy and is typically between about 40 to about 100 nanometers (nm) thick. The reflective layer is usually deposited by one of many well-known vapor deposition techniques such as sputtering or thermal evaporation. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed. Vol. 10, pp. 247 to 283, offers a detailed explanation of these and other deposition techniques such as glow discharge, ion plating, and chemical vapor deposition, and this specification hereby incorporates that disclosure by reference.
Next, a solvent-based or a UV (ultraviolet) curing-type resin is applied over the reflective layer, which is usually followed by a label. The third layer protects the reflective layer from handling and the ambient environment. And the label identifies the particular information that is stored on the disc, and sometimes, may include artwork.
The information pits residing between the polycarbonate resin and the reflective layer usually take the form a continuous spiral. The spiral typically begins at an inside radius and ends at an outside radius. The distance between any 2 spirals is called the "track pitch" and is usually about 1.6 microns. The length of one pit or land in the direction of the track is from about 0.9 to about 3.3 microns. All of these details are commonly known for compact audio discs and reside in a series of specifications that were first proposed by Philips NV of Holland and Sony of Japan as standards for the industry.
The disc is read by pointing a laser beam through the optical grade polycarbonate and onto the reflective layer with sufficiently small resolution to focus on the information pits. The pits have a depth of about 1/4 of the wavelength of the laser light, and the light generally has a wavelength in the range of about 780 to 820 nanometers. Destructive (dark) or constructive (bright) interference of the laser light is then produced as the laser travels along the spiral track, focusing on an alternating stream of pits and lands in its path.
This on and off change of light intensity from dark to bright or from bright to dark forms the basis of a digital data stream of 1 and 0's. When there is no light intensity change in a fixed time interval, the digital signal is "0," and when there is light intensity change from either dark to bright or bright to dark, the digital signal is "1." The continuous stream of ones and zeros that results is then electronically decoded and presented in a format that is meaningful to the user such as music or computer programming data.
As a result, it is important to have a highly reflective coating on the disc to reflect the laser light from the disc and onto a detector in order to read the presence of an intensity change. In general, the reflective layer is usually aluminum, copper, silver, or gold, all of which have a high optical reflectivity of more than 80 percent. Aluminum and aluminum alloys are commonly used because they have a comparatively lower cost, adequate corrosion resistance, and are easily placed onto the polycarbonate disc.
Occasionally and usually for cosmetic reason, a gold or copper based alloy is used to offer the consumer a "gold" colored disc. Although gold naturally offers a rich color and satisfies all the functional requirements of a highly reflective layer, it is comparatively much more expensive than aluminum. Therefore, frequently a copper-based alloy that contains zinc or tin is sometimes used to produce the gold colored layer. But unfortunately, the exchange is not truly satisfactory because the copper alloy's corrosion resistance, in general, is considered worse than aluminum, which results in a disc that has a shorter life span than one with an aluminum reflective layer.
For the convenience of the reader, additional details in the manufacture and operation of an optically readable storage system can be found in U.S. Pat. Nos. 4,998,239 to Strandjord et al. and 4,709,363 to Dirks et al., the disclosures of which are hereby incorporated by reference.
Another type of disc in the compact disc family that has become popular is the recordable compact disc or "CD-R."This disc is similar to the CD described earlier, but it has a few changes. The recordable compact disc begins with a continuous spiral groove instead of a continuous spiral of pits and has a layer of organic dye between the polycarbonate substrate and the reflective layer. The disc is recorded by periodically focusing a laser beam into the grooves as the laser travels along the spiral track. The laser heats the dye to a high temperature, which in turn places pits in the groove that coincide with an input data stream of ones and zeros by periodically deforming and decomposing the dye.
For the convenience of the reader, additional details regarding the operation and construction of these recordable discs can be found in U.S. Pat. Nos. 5,325,351 to Uchiyama et al., and 5,391,462; 5,415,914; and 5,419,939 to Arioka et al., and 5,620,767 to Harigaya et al., the disclosures of which are hereby incorporated into this specification by reference.
The key component of a CD-R disc is the organic dye, which is made from solvent and one or more organic compounds from the cyanine, phthalocyanine or azo family. The disc is normally produced by spin coating the dye onto the disc and sputtering the reflective layer over the dye after the dye is sufficiently dry. But because the dye may contain halogen ions or other chemicals that can corrode the reflective layer, many commonly used reflective layer materials such as aluminum may not be suitable to give the CD-R disc a reasonable life span. So being, frequently gold must be used to manufacture a recordable CD. But while gold satisfies all the functional requirements of CD-R discs, it is a very expensive solution.
Still another type of disc in the optimal disc family that has become popular is a prerecorded optical disc called the digital video disc or "DVD." This disc has two halves. Each half is made of polycarbonate resin that has been injection or compression molded with pit information and then sputter coated with a reflective layer, as described earlier. These two halves are then bonded or glued together with a UV curing resin or a hot melt adhesive to form the whole disc. The disc can then be played from both sides as contrasted from the compact disc or CD where information is usually obtained only from one side. The size of a DVD is about the same as a CD, but the information density is considerably higher. The track pitch is about 0.7 micron and the length of the pits and lands is from approximately 0.3 to 1.4 microns.
One variation of the DVD family of discs is the DVD-dual layer disc. This disc also has two information layers; however, both are played back from one side. In this arrangement, the highly reflectivity layer is usually the same as that previously described. But the second layer is only semi-reflective with a reflectivity in the range of approximately 18 to 30 percent. In addition to reflecting light, this second layer must also pass a substantial amount of light so that the laser beam can reach the highly reflective layer underneath and then reflect back through the semi-reflective layer to the signal detector.
Currently, the potential choice of the semi-reflective layer is either gold or silicon in the thickness range of 5 to 70 nanometers, as discussed in U.S. Pat. No. 5,171,392 to Iida et al., the disclosure of which is hereby incorporated by reference. Gold, when sufficiently thin, will both reflect and transmit light, has outstanding corrosion resistance, and is relatively easy to sputter into a coating of uniform thickness. But once again, it is also comparatively more expensive than other metals. Silicon is a reasonable alternative to gold, but because it is a semiconductor, its sputtering yield and sputtering rate are significantly lower than gold when applied with the same power. Moreover, silicon also has a tendency to react with oxygen and nitrogen during sputtering, which introduces a whole additional set of problems. For example, usually the application of silicon requires a more complicated sputtering apparatus than one that is normally required to apply other reflective metals. And as a result, neither gold nor silicon offers an ideal semi-reflective layer for use in this type of disc.
For the convenience of the reader, additional details regarding the manufacture and construction of DVD discs can be found in U.S. Pat. No. 5,640,382 to Florczak et al. the disclosure of which is hereby incorporated by reference.
Therefore, what is needed are some new alloys that have the advantages of gold when used as a reflective layer or as a semi-reflective layer in an optical storage medium, but are not as expensive as gold. This invention addresses that need.